Optoelectrical state indicators include an encoding disc that has an optically readable pattern. The disc pattern is read by one or more detectors which each deliver an electric signal in relation to the amount of light that is received in the detector, so that movement of the encoding disc in relation to the detector will be indicated by changes in the electric signal.
The encoding disc includes a periodic pattern, such as a plurality of light and dark fields of mutually the same size, for instance. When the encoding disc is twisted or rotated, the change between dark and light fields can be detected and a change in angle thus determined.
Optoelectrical encoders are often used in environments that are subjected to much vibration, and often in a heavily contaminated environment. The encoder may, for instance, be used to generate the real values of shaft rotation in a paper machine. For instance, the encoder may be hung on a rotating shaft in the paper machine. The encoding disc is caused to rotate as the shaft rotates, and the encoder casing and detector are firmly affixed to the casing of the paper machine.
One problem with the known encoders is that dirt is liable to enter the encoder unit and therewith soil the encoding disc and thereby cause the encoder unit to deliver an incorrect output signal. A further problem is that an encoder unit bearing can become worn after a long period of use, causing a change in the distance of the encoding disc from the detector unit, which, in turn, can result in disagreement of the encoder-unit output signal with the original pattern of the encoding disc.
U.S. Pat. No. 5,302,944 teaches an encoder that includes an encoding disc, a light source and two detectors. The encoder generates two staggered quadrature signals A and B in response to the detector signals. The output signal of the first detector is coupled to a clocked comparator which compares the voltage level of the first detector output signal with a reference level when a clock signal is received. The output of the comparator is supplied to a counter. One of the two quadrature signals is utilized to provide clock pulses for the sampling the other of the two quadrature signals at the peak thereof. When the first detector signal, at the clocked time, is lower than the reference voltage the counter will increase its count value, referred to as error signal e, and the drive voltage to the light source depends on the count value for the purpose of maintaining a certain detector voltage level also when the scale is dirty. Unfortunately the clock pulse signal for checking the A-signal will not be provided if the B-signal malfunctions, since the B-signal is used for the clock signal.
U.S. Pat. No. 4,827,123 describes an encoder that includes a pair of light sources and a pair of light detectors disposed on opposite sides of an optical shuttered disc. The shutters are comprised of two phase-shifted light tranmissive and light blocking areas which result in a change in the output signals from the detectors in accordance with the Gray Code as the optical disc rotates. According to U.S. Pat. No. 4,827,123, four states are defined that occur in a specified first sequence in one rotational direction, and a specific second sequence when the optical disc rotates in the opposite direction. A microprocessor analyses the generated state sequences and when none of the predetermined sequences occurs generates an alarm state indicating that a failure has occurred in one of the light emitters or light detectors or on the optical disc.